1. Field of the Invention The invention relates to a method of extruding a rigid plastic pipe of a thermoplastic polymer and controlling the wall thickness of the extruded pipe by means of a screw extruder provided with an extrusion die comprising a moulding end part, the temperature in each of several temperature-controlled sectors disposed about the moulding end part of the extrusion die being controlled as a function of the measurements of the wall thickness at a number of measuring points, each located in measuring sectors which lie in the axial extension of the temperature-controlled sectors and more particularly to a method of extruding a rigid polyvinylchloride pipe in this way.
By the expression a rigid plastic pipe is meant a pipe being flexible or not having such a wall thickness that this pipe maintains substantially its shape during storage at a flat support and does not collapse.
2. Description of the Prior Art
A method of this kind is known from Dutch patent application No. 72 09 444. According to this known method, the wall thickness of the pipe, at a point where an extruded plastic pipe has assumed solid form, is measured by means of a backscatter-type sensing device rotating about the pipe, whereupon the signals emanating from the sensing device are transmitted to a control unit. From this control unit, signals are sent to heat-supply or heat-removal elements provided in each of a number of individual temperature-control sectors disposed about the moulding end part of the extrusion die, thus effecting the temperature in each of said temperature-control sectors. As the flow rate of the melted plastic in the moulding end part of the extrusion die varies in accordance with the temperature, the thickness of the plastic pipe will consequently change in such a temperature-control sector area.
Said known method has the drawback that it is difficult to obtain a pipe having a thickness which is very even over the entire circumference of the pipe notwithstanding a regular distribution of the heat supply or heat removal elements about the circumference of the pipe.
This disadvantage is particularly important in pipes obtained by extruders having two screws which pipes comprise principally two opposite thickened wall parts extending in a longitudinal direction of the pipe.
In such cases these disadvantages cannot be removed by a better centering of the core in relation to the wall of the moulding end part of the extrusion die.